In a normally functioning heart, the sino-atrial (S-A) node generates electrical signals that control the contractions of the heart. These signals are generally in the form of quasi-periodic voltage impulses that are of sufficient magnitude to cause the contraction of the heart muscle. In a single cycle of the heart, a signal (i.e., a voltage impulse) is generated by the S-A node, causing the right and left atria to contract. The contractions of the right and left atria force blood into the corresponding right and left ventricles. The signal is also conducted through the AV node to the right and left ventricles (after a short time delay), causing the right and left ventricles to contract.
Various disorders in the cardiac electro-physiological system can cause abnormalities in the rate and/or the timing of the contractions of the heart. For example, a malfunctioning AV node conduction system can delay or prevent the transmission of the signal to the right and left ventricles, impairing or preventing the stimulation of the ventricles. Such disorders can often be corrected by use of a cardiac pacemaker.
Cardiac pacing involves the electrical stimulation of the heart in order to control the timing of the contractions of the heart. Electrical stimuli in the form of pulses are generated by a battery-powered pacemaker and applied to the tissue of the heart by one or more electrodes that are connected to the pacemaker via flexible, insulated conductors. The electrical stimuli supplement or supersede the electrical signals generated by the S-A node. The insulated conductors and associated electrodes form what is referred to as the "pacing lead." In addition to being used to apply pulses to the heart, the electrode (or electrodes) of a pacing lead may be used to sense intrinsic electrical activity within the heart.
Modern pacemakers commonly use a pacing technique known as "atrial tracking," wherein intrinsic electrical activity sensed within the right atrium is used to control the timing of pulses applied to the right ventricle. A pacemaker may be programmed, for example, so that each time an intrinsic electrical impulse is sensed in the right atrium, the pacemaker waits for a preprogrammed time delay (e.g., 75 milliseconds) and then applies a pulse to the right ventricle. Atrial tracking ensures that the ventricles contract shortly after each atrial contraction, and thus ensures that the atria and ventricles contract at the same rate.
Atrial tracking generally works well provided that the S-A node responds to the metabolic demand of the body by appropriately adjusting the rate at which impulses are applied to the atria. In certain situations, however, the intrinsic signal activity in the right atrium serves as a poor rate-setting mechanism for pacing the ventricles. Such situations include sinus bradycardia, fixed atrial fibrillation, giant silent atrium, and the bradycardia-tachycardia syndrome. Since disorders of this type are fairly common, it has recently become common to use an implantable physiologic sensor to provide information to the pacemaker that allows the pacemaker to estimate metabolic demand and adjust the pacing rate accordingly. This form of pacing is commonly known as "rate adaptive" or "rate responsive" pacing. Information provided by the physiologic sensor may also be used by the pacemaker to determine how the heart is responding to a particular pacing pattern. Further, information provided by the sensor may be conveyed to a doctor (using conventional telemetry techniques) to facilitate detection of abnormalities in the operation of the heart.
Various types of implantable sensors can be used to provide information to the pacemaker. One common type of sensor, for example, is a motion sensor that uses a piezoelectric device to sense low frequency motion associated with muscular activity. The output of the motion sensor is typically in the form of an analog signal, the amplitude and frequency of which indicate the degree of motion. When a high degree of motion is indicated by the signal, indicating increased physical activity, the pacemaker increases the rate at which pulses are applied to the right ventricle to increase the output of the heart. Other types of physiologic sensors that may be used by the pacemaker include, for example, blood pressure sensors, oxygen saturation sensors, respiratory-rate sensors, flexure sensors, flow sensors, carbon dioxide sensors, and temperature sensors.
It is known in the art to mount a physiologic sensor along a pacing lead in order to facilitate implantation of the sensor. The information signal generated by the lead-mounted sensor is conveyed to the pacemaker along a pair of conductors that are embedded within the insulating material of the pacing lead and which extend longitudinally through the lead. Each conductor is terminated at the proximal end of the lead with a connector pin that is adapted for insertion into a corresponding feed-through connector on the housing of the pacemaker, allowing electrical connection to be established between the sensor and the pacemaker once the lead has been inserted into the heart.
Existing physiologic sensors that are adapted for placement along the pacing lead are designed to communicate over dedicated pairs of signal conductors. Thus, in order to provide multiple sensors along a pacing lead, it is necessary to pass multiple pairs of signal conductors within the pacing lead--one pair of signal conductors per sensor. It is also necessary to provide a dedicated pair of connector pins and a dedicated pair of feed-though connectors for each sensor, and to provide a separate receiver circuit within the pacemaker for each sensor. These additional hardware requirements increase the cost and complexity of the pacing apparatus and the complexity of the implantation procedure, and have accordingly deterred pacemaker manufacturers from providing multiple sensors along a lead.
A need thus exists in the art for a system that allows sensors to be added to the pacing system without a corresponding increase in the number of conductors that must be passed through the pacing lead and connected to the pacemaker.